Recovery of fatty acids



April 9, 1940. T. F. MccoRMlcK Er AL RECOVERY OF FATTY ACIDS Original Filed Dec. 27, 1935 3 Sheets-Sheet lv PHE 9 3940. T. F. MCQQRMICK Er AL 96,423

RECOVERY 0F FATTY ACIDS Original Filed Dec. 27, 1935 3 Sheets-Sheet 2 3 Sheets-Sheet 3 Zg//w/e" d l. M n

T. F. MccoRMlcK Erm.

RECOVERY QF FATTY ACIDS original Filed nec. 27. 1935*' )Patented Apr, e), we@ A unirse `arras anar PATNT REVERJ @IF FATTE? ACHDS @riginal application December 27, 11935, Serial Divided and this application @ctober lll, 1939, Serial No. 299,554 e This invention relates to improvements in desulphurizing petroleum or its distillates and has for a particular object the removal of mercaptans from cracked distillate, especially cracked gasoline.

Another object of the invention is the reduction in, or elimination of, the usual sulphuric acid treatment used ordinarily in sulphur removal from cracked gasolines whereby the loss of valuable unsaturated hydrocarbons desired in the nished product is substantially prevented.

Another object is to increase the octane number or" cracked gasoline by removal or mercaptans.

Another object is the recovery of mercaptans in puried form and the recovery ci the treating agent so that a simplified process assures a minimum treating cost far below that oi heretofore known processes.

Another object is the recovery of a specific inercaptan in relatively puried form.

Another object is the recovery of lower molecular weight fatty acids-from a cracked distillate.

Another object is the elective removal of B2S coupled with stabilization of the distillate by removal of wild gases such as methane, etc.

Another object is to recover a commercial butane substantially free from HzS, from cracked products.

Further objects will become apparent as the invention hereinafter is more fully disclosed.

A feature of the invention comprises the removal of certain compounds contained in cracked gasoline which have a deleterious edect on the selective removal of mercaptans whereby the mercaptans are left in a condition easily susceptible of removal by themselves.

This will be better understood by stating that when mixtures of organic compounds, such as straight run gasoline. cracked gasoline and other hydrocarbon mixtures, are treated with an alkaline reagent, such as caustic soda solution, sodium carbonate solution and the like, a reaction with certain acidic compounds dissolved in the oil takes place.

Such acidic compounds may include phenols, naphthenic acids, hydrogen sulde gas, mercaptans, and particularly in the case of cracked distillates, fatty acids. Inasmuch as naphthenic acids, fatty acids, and HzS are relatively strong acids and are relatively easily removed from the (Cl. 26d-54m oil, such easy removal constitutes a base to work from leading to the selective removal of less strong acidic compounds such as mercaptans.

The primary step includes the substantially complete removal of HzS, preferably by the use of steps involving stabilization and fractionation ci the distillate under treatment, because it has been determined that the maior portion of the mercaptans present in gasolines, or cracked gasolines, is contained in a fractional part thereof, such as the rst 30% to-i0`% of the distillation range and, for instance, between 170 and 250 F., end boiling point and therefore a combined treatment is very effective for H28 removal and Stabilization for eliminating the use of chemicals.

Such fraction likewise contains a relatively high percentage of unsaturated hydrocarbons comprising olenes and aromatica which are especially desirable in the finished gasoline primarily on account of the antiknock value of the same and also by reason or their saving from loss during ordinary sulphuric acid treatment 'to sulp'nonation and polymerization.

In accordance with this invention, therefore, a gasoline or total cracked gasoline is split into two or more fractions, according to volatility, to yield a light :fraction comprising about to 50% of the cracked gasoline and a heavy fraction comprising fromabout 80% to 56%. This may be termed a split treat, and the said light frac-h tion may be treated for the removal and recovery of mercaptans so as to require a relatively light sweetening treatment thereafter.

'Ihe invention comprises alternative methods of treatment with respect to the H28 removal, in one of which the use of chemicals is eliminated and in the other of which a reduced chemical treatment may be used, but as .to certain novel steps of the invention, it is immaterial how the HzS is previously removed although a preferred form of operation without the use of chemicals will be particularly described.

While the treatment of cracked gasolines is also particularly described, it must be understood that other forms, or cuts, of petroleum distillates, coal tar distillates, shale oil distillates, or other hydrocarbons or mixtures of the same, are included in the scope of the invention.

In the appended drawings, Figure 1 is a descriptive flow sheet showing the treatment of both the said light and heavy fractions. 4

Figure 2 is a diagrammatic now sheet specifically illustrating the treatment of the said light fraction for removal and recovery of mercaptans, and regeneration of the treating agent.

Figure 3 is a diagrammatic flow sheet illustrating a preferred method of removing HzS from a distillate coupled with eiiicient fractionation thereof to provide the desired split stream for the desired light and heavy fractions which are thereby also stabilized.

ligure 1 is self explanatory and discloses the variations in treatments given the light and heavy fractions.

In Figure 2 the treatment of a light fraction is seen to comprise a general method of treating and includes the passage of total cracked distillate from any well known cracking and fractionating system 5 through a stabilizer'column 8 wherein a separation of so-called wild vapors occurs, such as H2B gas, methane, ethane, propane, etc., said vapors and gases being eliminated through valve 'I.

The liquid distillate, comprising a total cracked gasoline, then passes through pipe 8 into a still, or fractionator 9, wherein a desired cut is made preferably to yield a light fraction of about 20% to V50% of the distillate and a heavy fraction of about 80% to 50%.

The heavy fraction is withdrawn from still 9 through valve I tp be treated according to the procedure of Figure 1, while the light fraction containing the major portion of the mercaptans is passed through pipes II and I2 into the lower part of a contact tower I5, substantially filled with contact material I 8 comprising Raschig rings, glass beads, broken glass, crushed gravel or other suitable contact material supported on a suitable screen I'I. At times it may be desirable in the selective removal of organic acids to add heat to the distillate, in which case a heater I3 may be used. Pipe I4 is used for the supply of stabilized and purified distillate when the method of Figure 3 is used.

The light fraction owing upwardly through tower I5 bubbles through a body of caustic solution maintained at a level A-A, contact being enhanced by said Raschig rings etc., which contact is usually sumcient for the purpose oi the process. However, additional contact efficiency values may be obtained by circulating caustic by means of pump 23 from the bottom oi tower I5 through line 22, and dispersing the same over the top of the tower packing I6 by means of a spray I8 which causes the caustic to flow downward by gravityA countercurrent to the gasoline stream. When the free caustic value oi' the treating agent is reduced to a low figure, the spent solution is discharged through line 24 and represents a basic stock for the recovery of alkali soaps or free organic `acids as by-products. Partially spent caustic from a second contact tower 20 is pumped by pump I9 through lines2l and 22 to tower I5 and fresh caustic is charged through valve 28, line 21, and spray 28 to tower 20. If desired to maintain a certain temperature of treating in tower I5, above atmospheric temperatures, a heater 2Ia nay be cut into line 22.

The once treated light fraction from tower I5 [lows through line 25 to the second contact tower l0 (of similar construction to tower I5 and hayng a reagent level B-B) wherein it is contacted n similar fashion with fresher caustic solution supplied as' described through valve 28, line 21,

and spray 26. Continuous circulation of caustic solution from the bottom of the tower 20 through spray 26 by means of pump I8 will increase the contact andeillciency of treatment and may be Y used. 'I'he treating agent introduced through valve 28 may comprise a caustic soda solution of about 5 to 20 B, gravity.

'I'he above mentioned step countercurrent method is preferred but it is to be understood that a true countercurrent method may be employed. In such cases fresh caustic of from 3 to 20 B. gravity is charged continuously through valve 28, line 2'I, and spray 26 to tower 2li and hows downward by gravity countercurrent to the gasoline stream, contact taking place in the Raschig rings. In such processing no caustic level is held in tower 20, caustic being pumped by lpump I9, with up to 100% of the gasoline charge to the treating system, through lines 2| 22 and spray I 8 to tower I5 wherein contact takes place in the Raschig rings by gravity counterflow. A caustic level E-E is maintained below the gasoline inlet to this tower I5. The treating agent is charged through valve 28 in amounts sumcient to accomplish substantial removal of organic acids and is withdrawn continuously through line 24. As in the preferred system the treatment may be performed hot in which heaters I3 and 2Ia are employed but in general practice the treatment is performed at ordinary atmospheric temperatures.

'I'he treatment in towers I5 and 20 is used to take advantage of the selective removal of organic acids because it has been found that these compounds along with the mercaptans go into solution in fresh strong caustic soda solution but, as the soda solution becomes increasingly spent by absorbing, or reacting, with such compounds, the stronger acids replace the mercaptans, which are thus liberated to again go into solution in the gasoline.

While it is true that some mercaptans are removed in towers I5 and 20, the major portion of the mercaptans thusremain in solution in the light fraction and the amount of such removal is the result of an equilibrium condition in which the removal depends on the relative solubilities of mercaptans in the gasoline and caustic solution respectively. This distributional factor will be affected by the strength of the caustic solution in free alkali, the temperature, the quantity of the treating agent, and the concentration and type of mercaptan present in the gasoline, so that in providing initially a certain strength of caustic soda solution through valve 28 these facts must be borne in mind to effect minimum removal of mercaptans in towers I5 and 2l and a maximum recovery of stronger organic acids which represent a valuable by-product and which are withdrawn through line 24 when the free alkali content is reduced to a low ligure. The removal of strong organic acids in one or more steps is effective inf preventing or minimizing the quantity of such compounds which otherwise would be removed with the mercaptans in a later step of the process.

The spent caustic solution withdrawn through line 24 contains the stronger organic acids in form of sodium salts. In order to recover these acids the solution may be acidied, whereby the organic acids are set free and subsequently can be separated into chemical individuals by fractional distillation. Another method of purification and separation of those organic acids would be a conversion of the sodium salt mixture into alkaline earth or other metal salts, followed by fractional crystallization of those salts and nally regeneration of the individual acids from the separated salts. It is also possible to accomplish separation by esterifying the organic acid mixture and fractionating the resulting esters. It is found that the organic acids thus obtained belong to the lower molecular group of monocarboxylic acids, or fatty acids. Propionic and butyric acid are predominating in quantity,but the presence of lower and higher molecular homologues of this group is indicated,

These fatty acids are very valuable raw materials for the chemical industry, where they find widespread use in the manufacture of esters, dyestui intermediates and pharmaceutical products.

This equilibrium reaction is especially valuable in treatment of the light fraction derived from the above described "split-treat because it is found that not only is the major portion of the mercaptans from the total unsplit cracked gasoline concentrated in the said light fraction cornprising say up to about 40% of the gasoline, but the low boiling mercaptans within the boiling range of the light fraction are the ones most soluble in caustic soda solutions.

Hence, by fractionating out the low boiling constituents of a gasoline, or cracked gasoline, up to about 40% thereof and treating this frac'- tion separately with caustic solutions, a maximum removal of mercaptans is assured together with maximum desulphurization.

And, not only are the above factors important,

abut the removal of mercaptans by themselves relieves the load ordinarily thrown on the doctor treatment for sweetening wherein the mercaptans are altered to disulphides, and also the caustic solution is capable of regeneration for reuse for several cycles.

ln addition to the above described treatment for selective removal of relatively strong organic acids and HzS whereby a large percentage of the mercaptans remain dissolved in the light fraction, it is found that temperature has a marked eiect on the above equilibrium condition and that as the temperature of the mercaptan containing light fraction mixed with caustic soda solution approaches the freezing point of the said solution the solubility of the mercaptans in caustic increases in proportion.

In consequence, the treated light frac-tion issuing from tower 20 is passed through line 29 to a third similarly constructed contact tower 30, being cooled by cooler 3i to a point preferably just above the freezing point of a caustic solution introduced into tower 30 by pump 32 from the bottom of a fourth similarly constructed contact tower 38 through lines 33, 3ft, and spray 35.

The caustic solution then passes in true countercurrent ow to the light fraction through tower 30, the spent caustic containing mercaptans being withdrawn to tank 36, while the light fraction relieved lof part of the mercaptans ows through line 3l to a fourth similarly constructed contact tower 38, being cooled, as before, on its way by cooler 39 to a point preferably just above the freezing point of fresh strong caustic solution supplied through line 40, cooler 13|, and spray $2.

In tower 38, contact of the once treated cooled light frac-tion is completed with fresh caustic to finally remove all mercaptans in the caustic in the form of sodium mercaptides as far as is possible under the optimum conditions of temperature and strength of soda solution.

En order to assure the most efiicient contact, pump 32 provides a rate of circulation from tower 3% back to tower 30 at a rate up to about 50%, or higher, of the volume of the light fraction under treatment, which is finally withdrawn through line i3 to undergo the usual doctor treatment for sweetening and is then a finished light fraction ready for blending to yield a nished motor fuel. Some of the advantages of thus treating the light fraction may be summarized as follows:

(l) The unsaturates present in the light fraction which are highly desired for their antiknock value in finished gasolines are not attacked and lost as in sulphurlc acid treatment.

(2) .A re-run distillation with its accompanying losses may, in many cases, be dispensed with, clue to the elimination of polymerization which takes place in ordinary acid treatment.

(3) The loss in octane number is not only minimized when compared -with acid treatment followed by distillation but the octane value may be actually increased.

(4) The desulphurization by mercaptan removal is a maximum with the selective treatment of alight fraction.

(5) The maximum value may be obtained from the caustic soda solution by regeneration and reuse.

The temperature to which the reacting mixture Ais reduced in towers 30 and 33 may be just above the freezing point of the caustic solution used up to 60 F., and the amount of caustic soda solution used ranges from about 5% to 30% by volume of the light fraction treated and is oi a strength from about 3 to 20 Be. Preferably, the caustic strength used is between 5 and 12 B., and about by volume and the treating temperature in towers and 33 and 30 to 40 F.

In cases where a more selective removal of strong organic acids is desired, a hot caustic treat is used in tower l5 by means of heaters i3 and 2id. The solubility of mercaptans in the caustic solution increases as the freezing point of that solution is approached so that as the temperature rises the quantity of mercaptans extracted with `the organic acids is reduced. This treatment is preferred in the liquid phase and applies to both the light and heavy fractions. rTemperatures up to 300 F. and pressures up to 300 pounds per square inch, absolute, may be employed and caustic strengths from 5 to 30 B. utilized. In general, however, it is preferred to treat at normal atmospheric temperatures which range from 30 to 110 F. and use pressures up to 100 pounds per square inch, absolute, or higher, which pressures may be applied by the use of suitable valves in the system.

Referring now to Figure l, it will be seen that the heavy fraction from still 9, which contains a small amount of mercaptan sulphur, for example, of the order of 0.08%, may be given a caustic soda wash to remove relatively strong organic acids and mercaptans according to the steps already disclosed in treating the light fraction for the same purposes.

Although the heavy mercaptans are much less soluble in caustic soda than the lower molecular weight compounds, a partial removal is eifected, thus reducing the mercaptan content of said heavy fraction. Such processing is beneficial when high acid rates are not required, for example cn pressure distillates produced from selected cracking stocks which yield cracked gasolines of low sulphur content. The spent caustics derived 'from this mercaptan removal are solutions of the sodium mercaptides of higher boiling members of the mercaptan group and represent a further separation of selected mercaptans for by-product recovery.

5 The heavy fraction thus relieved of organic acids and mercaptans is then treated in countercurrent iiow with concentrated'sulphuric acid of say 66 Baum to 98% strength, or higher or lower, at a preferred temperature of from F. to about 50 F. to remove such sulphur compounds like thiophenes which are not removable by caustic soda solutions.

After drawing oi the acid sludge, the heavy fraction is redistilled, then doctor treated to yield iB a finished heavy fraction ready for use as such, or for blending with the light fraction treated as above described to yield a finished gasoline, or motor fuel.

' At times, it may be desirable to give the light l0 fraction passing from tower 38 through line 43, a slightl acid treatment to stabilize the gasoline for color and to remove gum forming compounds, or to further desulphurize by removing other kinds of sulphur compounds.`

l This may be accomplished by contacting the light fraction, after removal of mercaptans as described, with the acid sludge derived from the treatment of heavy fraction, or with fresh acid 0 of a suitable degree of concentration at a suitable rate.

After thus acid-treating, the light fraction may be re-run and doctor treated to be then blended with the treated heavy fraction to form a total finished gasoline.

5 It is apparent that, after separate acid treatments of the light and heavy fractions, the fractions may be blended and then re-run and doctor treated to obtain the finished motor fuel.

In the acid treatments of the light and heavy n fractions as described above, it is preferred to use as the most emcient and economical method the processes described in aco-pending application Serial No. 50,206, filed November 16, 1935, and in U. S. Patent No. 2,052,852 issued to Edwards and y Stark on September l, 1936, which show the true countercurrent contacting desired, but it should be understood that the herein described invention is not limited to any particular type of contacting with acid, or caustic, or to the degree of concentration of either, or to any particular kind of hydrocarbons to be treated, as the disclosure comprehends the treatment of any kind of hydrocarbon containing mercaptans in accordance with the described steps, preferred forms only being s illustrated herein. Caustic soda solution may be replaced by caustic potash, as is well known.

The efficiency of the process as a whole, while not dependent thereon, ismaterlally increased by the regeneration and reuse of the spent caustic g solution from( tower II, andthe regeneration may be accomplished by either batch or continuous operation. While .batchregeneration is effective, continuous 'regeneration is to be preferred in order to conform with the continuous l operation of the preceding steps set forth.

In the batch regeneration.- by reference to Figure 2, it will be seen thatthe spent caustic containing sodium mercaptides is drawn from tank 35 through line 44 to tank 45 containing a i closed steam coll 45 and an open line 41 for the introduction of air, steam, or an inert gas for stripping.

A solution level X-X gives head room for the removal of the low boiling mercaptans in vapori form throughipipe 48, the same being derived by the splitting up of the sodium mercaptides under the inuence of heat applied through coil 45 to bring the solution to or near, the boiling point.

Alternative methods of applying open steam, air, etc., may be used. If steam alone be passed through the mixture, the mercaptan sulphur may be reduced from say 50 grams per liter to as low as 2 grams per liter. If the velocity of the steam through the mixture is great enough to cause foaming, additional steam may be passed through spray line 49 to break the foam.

Alternatively and preferably, continuous regeneration is effected by passing the spent caustic solution from tank 35 through line 44 and pump 50 to the top of a contact tower 5l containing a plurality of bubble plates 52, the caustic solution nowingdown countercurent to the heating and i stripping steam or gas supplied through line 53.

In such a contact tower 5|, when insulated and being 4' in diameter and 32 high, using 1.8

pounds of steam for 5 pounds of caustic solution e charged, it is found that with seven plates the mercaptan sulphur content will be reduced from grams per liter to 6 grams per liter. Using a higher ratio of steam to caustic the mercaptan y content may be still further reduced, onthe number of plates may be increased to provide an ad ditional reduction in mercaptan sulphur.

The mercaptans are taken as overhead through line 54 to an insulated slop pot 55, wherein a separation of vapors from any caustic in the form of foam takes place, and the mercaptan vapors then pass through line 51 to a contact tower 58 containing suitable contact material 59 in which the vapors are scrubbed with water applied through pipe to condense the mercaptans at I a predetermined temperature and pressure.

Vapors are tapped oil' through line 5| and are composed largely of the more volatile mercaptans as methyl andA ethyl mercaptan. These mercaptans may be recovered by suitable absorption agents or may be liquefied by means of refrigeration orcompression in the well known manner. Mercaptans condensed by the water pass to a settling tank 52 wherein the water separates by gravity to be withdrawn through pipe 63 controlled by an automatic liquid level regulator 64. 'I'he desired liquid mercaptans are drawn oil.' through pipe 55.

Any caustic solution passed over from tower 5I and trapped out in the slop pot may be retin-ned by pump 55 for restripping in tower 5|'.

The regenerated caustic solution ilows continuously from tower 5l into tank 55'whence it may be withdrawn through line 51 to be used as charging caustic through line 40. In similar fashion, the regenerated caustic solution from the batch treatment (when used) may be passed through line 40 to line 40. I

The life of the regenerated caustic solution is not necessarily indefiniteas there may be a gradual accumulation of impurities in the same which are not responsive -to'regenerative separation,

but it may be used effectively in towers 38 and 35 for at least three cycles which gives great economy in caustic consumption.

Referring to Figure 3, a preferred method of removing H28 from a raw cracked distillate, while stabilizing the same during fractionation to obtain the before described light and heavy fractionsis shown. f,

In these steps, which are preferably used in place of the stabilizer column 5 and still 9 oi'v Figure 2, the flow sheet and data represent actual u assaggi renery operations during the production of on ished cracked gasoline from a raw cracked distillate having the following characteristics:

Gravity 58.2 Sulphur ;oer cent 0.75 Octane number (CFR. motor method) 70 Initial B. point 90 per cent 112 l0 do 124 do 160 do 191 l0 do 227 ,do\ 25'? do 283 do 309 do 343 9o do 366 95 do 39s End point 399 Recovery.. 95.0 Residue 1.0 Loss 4.0

The raw cracked distillate from the cracking stills is forced by pump l0 through line 'll into a lash tower l2, absorbing heat during its passage from partial condenser 7S, heat exchangers M and heater l5 so that the distillate is discharged into flash tower l2 at about 362 F., or at a temperature suiicient to release H2S, wild gases and certain desired low boiling fractions.

Flash tower 'l2 operated successfully and einciently in the present invention with nine plates, but any desired number can be used, and with a suitable reflux` liquidwhich may conveniently be supplied from another part of the system by pump l@ and line lll, to separate up to say 25% of low boiling constituents together with the greater portion or the H25.

Operating flash tower l2 with entering dis-1 tillate at 362 il. and under a pressure ci 25 pounds per square inch, gage, the desired light fraction is taken o overhead with the H25 which latter will then Toe found to be reduced the be tonus which tllough line "lo passed into a lil. Said bottoms are injected at a temperature of about tion by removal oi the rem light ends oi the boiling range it fraction.

heated about 375 F., ope sure about lil pounds per square ie overhead from primary tower 7S which passes thereto way of line and ,l condensers lf3 and SS.

In ary tower i9, operating under the above described conditions, the H25 will vbe reduced from @.09 gram per liter or" the entering stools to a trace in the rectlecl heavy fraction passing out through line lo to the treating system described, while the overhead (comprising constituents of the desired light fraction) passes from accumulator' i2 through line 05 and condenser l to be collected in accumulator 3S in liquid form, incondensible gases being taken 0E through line e@ for recompresslon, absorption, or other uses.

The condensed overhead is picked up by pump through line lil?, or may be passed at a temperal@ and passed through line and heat erf changer 9! at a temperature of about 160 li'. into a 34 plate secondary tower 92, being joined therein and in its passage by the vapor overhead brought from ash tower l? through line 93. 5i

Secondary tower 92 is provided for the pur pose of stabilizing the desired light fraction by the elimination of undesired wild gases such as methane, ethane, etc., during which operation all H28 is separated. To this end, tower @2 is pro-- il@ vided with a reboiler @fl adapted to heat the bottoms to a temperature or" about 300 F. thereby giving a temperature gradient through the column and yield an overhead having a temperature of about 120 F. at the outlet pipe 95 while operating under a pressure of about 245 pounds per square inch, gage.

The overhead from secondary tower E taken through line and condenser il@ to pro= vide condensate in accumulator which E forced by pump 03 through line 9@ as reflui: in the upper part of the tower. Excess condensate may be taken through line l0@ to be used as blending propane, or for other purposes, while the incondensible wild gases and B2S are withu drawn through line lol.

Depending on the usev desired for the light fraction as to boiling range, etc., it may be with@ drawn as is for the described caustic treatment ture of about 300 F. through line los into a 2e plate butano tower lill wherein a out of commercial butane is taken as overhead, the bottoms comprising the desired light fraction within the gasoline boiling range being withdrawn through lines 05 and E02 for the described caustic treat ment.

When a cut comprising substantially butanes and hydrocarbons of similar boiling range is de sired with, for example, ah absolute vapor sure or" about 2e to 75 pounds per square in at 70 F., tower lilo may be provided with a c boiler l0@ giving a bottoni. t perature of about 220 l5. and. a. temperature outlet line lo? o. about F. while opera' under pressure of f about 80 poundsperso `In, gage. Such a out will coi rocarbons con ist chiefly ci :four carbon atoms to ti e molecule, su s .exa-noie normal 'ana normal butylene, isooutylene, and isobutane. The following is typical ezrar e o' a ci however e portions of Le various .c 1.7 with the ty e ci crocl stock; to the craclmg cetera: f3 Propane @.Zroolf't Isobutane 22.3 l. .isobutylene 15.2 mol /o Normal butylene 23.5 mol. m Butadiene 2.0 mol. W Normal butano 33.1 mol. Methyl mercaptan @.3 roel. Isopentane and heavier 3.1 mol.

The overhead passing through line lill, reilux. e condensers Mld and head condenser im, will new as a liquid through line ilo, part being used as reflux through line l l l and the remainder dowing through line H2 and coolers i118 to a treating system for the recovery of methyl mercaptan in e@ substantially pure form. The incondensible gases are withdrawn through line Ml.

1t is :found that by thus running a raw cracked distillate, as described, when o. butano cut is taken Inercaptans are present up to about 0.35% by im weight mercaptan sulphur and that the mercaptan sulphur is derived from the specific mercaptan, methyl mercaptan CHzSH, which is removed by caustic soda solutions according to the following equation:

yIn consequence, the butane cut from tower IM through line H2 may be treated with caustic soda solution according to the steps outlined with respect to the operation of towers 30 and 38 in Figure 2. In this case, towers 30 and 38 will be supplied with cooled caustic soda solution of about to 12 Baum strength and the cooled butane will ow countercurrent to the caustic with eflicient contacting due to the structure of the towers, the reaction being conducted at about 30 to 40 F.

The spent caustic may then pass, as'described in connection with Figure 2, from tower 30 to the described continuous regeneration plant when pure methyl mercaptan may be recovered from line 65. v

More simply, the methyl mercaptan may be removed from the spent caustic by boiling the latter and condensing the distilled mercaptan, or by passing steam, air, or other inert stripping gas through the hot caustic solution and condensing the distilled mercaptan.

The preferred system, above outlined, for obtaining desired light and heavy fractions for separate treatments in the removal of impurities has many advantages. 1

It provides for the complete removal of lHzS without recourse to the chemical treatment' usually necessary.

It gives improved fractionation for the desired splitl wherein gaps ofover 20 F. between the A. S. T. M. Engler end point of the light 'fraction and the Engler initial boiling point of the heavy fraction can be obtained, which is important when a concentration of mercaptans has been determined in a given percentage lof the raw distillate and thus limits the low boiling sulphur compounds to the light fraction.

It gives by improved fractionation a light fraction of a desired boiling range which Vislargely uncontaminated with higher boiling and more refractory types of sulphur compounds or with compounds which may impair color stability.

It gives a light fraction free from I-IzS of such character that the rst treatment thereof with warm or hot caustic solution permits the selective removal of organic, or fatty, acids uncontaminated with HzS reaction products.

It minimizes the use and cost of chemicals while yielding relatively pure valuable by-products. l

It gives a commercial butane free from HzS.

It yields a speciiic mercaptan in relatively pure form.

It provides ultimately for a puried cracked gasoline of superior qualities'of octane rating and color stability. Combined with these is the recovery of impurities in a valuable commercial form and a high reduction in treating costs.

It must be understood that the preferred method of Figure 3 has been applied to the treatment of a cracked gasoline specically for the purpose of illustration and not of limitation. inas .much as lvariations in temperature, pressure,

tower construction and the like are permissible depending on the required split in the boiling ranges of the raw cracked distillate, the character of the cracked gasoline and many other variables Well known in the art.

For instance, in the removal of HzS alone, the operation of flash tower 'I2 is not as important as its chief function which is the stripping of the 5 lightest hydrocarbons from the raw distillate so that the load on the primary tower 19 is reduced and thus permits condensation of overhead vapors therefrom at ordinary cooling water temperatures and comparatively low pressures. This permits the use of a less costly primary tower 19 and cuts down the use and cost of steam used in reboiler 80 and the cost of condensing the overhead vapors .with cooling mediums other than Water. I

Again, while correct control and adjustment of .the operations in primary tower 19 and secondary tower 92 are important for the removal of HsS the gures given for a particular cracked distillate may be varied to suit conditions. For 2q instance, if it is desired to operate these towers at higher pressures, the top and bottom temperatures must be raised or, vice versa, i1' the pressure is reduced, the temperatures should be reduced.

A great many of such variables follow physical 2l laws, the operation of which is well known to those skilled in the art, and the practice of the invention requires no other instructions to permit its use beyond those given. Ordinary laboratory controls to determine characteristics of a particular distillate will govern correct reilnery control of the process.

The increase in octane number is another result of the invention.

In the usual sulphuric acid treatment of cracked distillates for the removal of sulphur A compounds and other impurities it is possible to y accomplish complete mercaptan removal with acid but due to resultant large losses such treatment is usually uneconomical.

degree of total lsulphur removal and the mercaptans which remain after such treatment are often from 10 to 50% of those -present in the original stock. These compounds must be converted to disuldes by a sweetening process such as plumbite treating, in order vto improve the odor`of the product and meet specifications.

By `such usual treatments lthere is often a drop in octane value (as determined by the C. F. R.

motor method) as high as 4 to 5 octane numbers under the original stock with a corresponding derogatory eiect on the lead susceptibility of the gasoline. While such drop, or loss. is partly due to the formation of disulphides during doctor treatment. yet octane number losses occur in such treatment over vand above what is ex pected by the complete conversion of mercaptans to disulphides. 5 In consequence, it is highly desirable to not o0 only eliminate sulphuric acid treatment with itsl l methods, and temperature.

While it is true in average commercial use of# Just sulcient 4g acid is therefore added -to accomplish the desired the process, not tvarranting the ultimate purity of complete mercaptan removal, that a small per= centage of mercaptan remains in the gasoline after caustic treatment and provision is made for economically reducing this, yet the harmful eect of the remainder is markedly less than that in other known types oi mercaptan removal processes.

This is more particularly shown in the present invention wherein the light fraction, which contained 0.35% of mercaptan sulphur before the described caustic treatment, contained out 0.04% of such sulphur after caustic treatment.

At the same time, the removal of mercaptan sulphur, as described, increased the octane nurnber of the light fraction from 71.0 to 72.5 before and after caustic treatment respectively.

Thus, compared with the usual sulphur-ic acid treatment an increase in octane number is se cured instead of a loss.

The use of doctor solution herein is principally for a safety factor because any slight traces or" mercaptans might be corrosive. Further, the malodorous mercaptarls highly oensive ih commercial fuels.

Hence, while the mercante-ns may not oe elimihated to an ultimate degree in the system withm out doctor treatment, the use of a doctor treatment on largely mercaptan'ree distillate assures a minimum. conversion or mercaptans to disul-za phides with accompanying loss of octane rating, While effecting removal of the main body of cor= rosive and bad smelling compounds.

The entire process, as described, as a Whole comprises a number or" steps which, both alone and in combination in one form' or another, give .maximum results desulphurization, minimum losses of valuable hydrocarbons, high economy ih treating costs, and i roi/ed recovery of valuable bgd-products.

These steps r'eay ed trom cau. ic sou the ing caustic solution ed use.

to nciht the fraction yields acid ype, like propiohio butyrie a o a mhiilnuro contaminan tion with ehehe. aphtheuie acids, latter heil at e range.

iutiohs and available for co (e) The low lo' organic acide Coreprises the selective removal ci and relatively strong organic acids from the light fractions prior to selective mercaptan removal. This is important because,

(c) it prepares light fraction for selective 'the caustic solution.

mercaptau removal by the elimination o acidic compounds which would prevent such selective removal.

(b) The elimination or" strong organic acids reduces the foaming tendency of the spent caustic solutions derived during this selective mercaptan removal step, during regeneration. y

(c) The strong organic acids removed represent a source of valuable lay-products.

Step N0. 3

Comprises the cooling of the light fraction to a temperature just alcove the freezing point of Under average circumI stances however, we preier to treat at tempera,- tures of Sil-'10 F. This step the advantage of,

(a) Giving the greatest .rate of .mercaptah re-1 maval with a 'rate oi caustic supply of a given concentration. v

(o) increases the rate of mercaptari removal when compared with higher temperatures.

Step No.

that the sulphur percentage is reduced to a Ligure unattainable with. suljohuric acid treatmeut without excessive losses.

Sie@ No. 5

comprises the regeneration of used caustic solution whichA is possible due to the selective removal of H25 and strong organic acids and gives the advantage of,

(o) Recovery oi sulosto tialy pure mercaptahs. (o) Recovery of regsherated solution or together, coth rei-cove compounds joelyme iight ctioo has not .oe

:1e-rin distillation may he eliminated with cor.-n eequerlt saving.

Stego No.' S

doctor, or other, sweetening wher; necessary. This step may ce eliminated when certain stocks are treated iu which the mercapn removal icy tl'le above described steps is such the sour sulphur coutent is lot-v.

The denition or the term total cracked distillate used herein indicates a distillate derived from cracking various stoclss and is usually a distillate cut to an end goomt by A. S. il?. M. die@ tillation similar to normal motor fuel marketed. However, the total cracked gasoline may have an end point from 250 F. to 500 F., or higher, de-` pending entirely on the particular fractionating process and/or equipment used.

As a rule, the end point on total cracked gasoline is cut with modern methods and equipment to somewhere between 350 F. and 437 F., but variations from this range are permissible within the scope of the invention and may include distillates from other sources such, for instance, as those made by polymerization of hydrocarbon gases as well as sulphur bearing distillates from other sources.

Among outstanding features of the invention are included two stages of caustic treatment, the first comprising caustic treatment at atmospheric or more elevated temperatures for the recovery of organic acids, the second comprising caustic treatment at reduced temperatures below atmospheric, or at effective low temperatures, for the removal of mercaptans.

In order to obtain these by-products in the most puriiied form, hydrogen sulphide should be v rst removed as described so that the selective removal of the other impurities may be readily accomplished.

application is a division of our copendin appplication Serial No. 56,398, iiled DecemberA We claim: 1. A process of recovering products from petroleum oils which comprises: treating a hydro- 3. A process of recovering product s from pe-v troleum oils which comprises: treating a hydrocarbon distillate containing aliphatic monocarboxylic acids and mercaptans with an alkali solution while at a temperature between the freezing point of. the solution and 300 F. suiliciently high to selectively absorb said aliphatic monocarboxylic acids and to leave said mercaptans dissolved in said distillate, removing and acidifylng y lsuch alkaline solution to liberate the absorbed aliphatic monocarboxylic acids, and fractionally distilling the liberated acids to isolate at least one aliphatic monocarboxylic acid.

4. A process of recovering products from petroleum oils which comprises: treating a hydrocarbon distillate containing aliphatic monocarboxylic acids and mercaptans with an alkali solution while at a temperature between the freezing point of the solution and 300 F. suiiiciently high to selectively absorb said aliphatic monocarboxylic acids and to leave said mercaptans dissolved in said distillate, removing the alkaline solution together with the resulting alkali salts of said acids, converting said alkali salts into alkaline earth metal salts, fractionally crystallizing said alkaline earth metal salts to isolate at least one individual salt, and recovering the aliphatic monocarboxylic acid from said individual salt.

THOMAS F. MCCORMICK. ARTHUR LAZAR. 

